In a number of optical systems, including those used for high energy laser applications, a beam control system is required to correct a wavefront for wavefront errors generated internally by the system, as well as for external disturbances. The wavefront errors can be large in magnitude and can exhibit a varying spatial and temporal frequency content.
As an example, in airborne laser applications the large magnitude/low frequency errors can be attributed to a boundary layer near the conformal window, while the small magnitude/high frequency (spatial and temporal) error can be attributed to atmospheric turbulence.
As another example, in a wide field of view (WFOV) application the large magnitude/lower frequency wavefront error can originate in a field-dependent WFOV beam expander, while the smaller magnitude/higher frequency wavefront error may originate in the laser and beam control system.
Referring to FIG. 1, a conventional technique to compensate for these wavefront errors employs two deformable mirrors 1 and 2. Mirror 1 is used to compensate for the large magnitude/low frequency error (low bandwidth (BW)), while mirror 2 is used to compensate for the smaller magnitude/higher frequency error (high BW). Deformable mirror 1 includes a base plate or backup structure 3 that supports a plurality of first actuators 5, which in turn support a facesheet 4 having a reflective surface 4A. Deformable mirror 2 is similarly constructed to include a backup structure 6 that supports a plurality of second actuators 8, which in turn support a facesheet 7 having a reflective surface 7A.
In general, the first actuators 5 will provide a larger range of linear motion (wider dynamic range), but with longer response time (lower BW), than the second actuators 8. A beam 9 to be wavefront corrected must therefore be directed so as to impinge on both surfaces 4A and 7A, with the large magnitude/lower frequency wavefront error being corrected by the low BW mirror 1, while the smaller magnitude/higher frequency wavefront error is corrected by the high BW mirror 2.
It can be seen that this approach to wavefront correction increases the complexity, mass, volume and cost of the system by requiring two mirror structures, with additional transfer optics (not shown) for directing the beam between the two mirror structures.